This invention relates to an electrooptical distance measuring apparatus.
In the prior art electrooptical distance measuring apparatus using the propagation velocity of light in space or in air is well known in various types. Most broadly used are instruments with sinusoidal modulation of the brightness of a light beam. After having travelled twice the measured distance up to an optical reflector and back again said modulation undergoes a phase shift which is measured by optical and electric means in dependence of the distance. After recent progress in the development of electric time measurement techniques the time of propagation of single light impulses or flashes also has been measured once or repetitively for determining distance. Up to now less frequently used are instruments for distance measurement according to the so called toothed-wheel method (A. H. L. Fizeau, 1849). Originally this method consisted of periodically interrupting a beam of light by means of a toothed-wheel, transmitting the interrupted beam to a reflector and after retroreflection periodically interrupting said beam a second time by the same toothed-wheel. Due to its retardation, the light beam, with a convenient number of revolutions per minute of the toothed-wheel, on its return will hit a tooth instead of a gap and be thus blocked from obervation. From the number of revolutions for this case the time of travel of the beam is calculated.
According to the state of the art, electrooptical crystals are used instead of the toothed-wheel (see U.S. Pat. No. 3,424,531 to P. L. Bender et al.). Such crystals instead of interruptions produce a periodic modulation of elliptical polarisation of the light beam.
A linearly polarized beam with suitable orientation of its plane of polarization with respect to the axes of the electrooptic crystal is modulated with a sinusoidal electric signal of some 100 MHz. If retroreflected beam components upon their second pass through the crystal in reverse direction meet the same phase of modulation as on their first pass, the original steady state linear polarization is restored and behind a suitable optical analyzer complete darkness of those beam components is observed. This is the case when at each moment the total number of modulation wavelengths present over twice the measured distance from the crystal to the retroreflector and back is an integer number. If it is not, the brightness of the beam will not be minimum, but then a minimum may be obtained by changing the measured distance or the wavelength of modulation. Both methods are state of the art (see also GB Pat. No. 919,368 to K. D. Froome et al.).
With one known apparatus for distance measurement of the above type, the modulating crystal is made from KDP, which needs a rather high a.c. voltage for modulation (see F. S. Chen, Modulators for optical communications, Proc. IEEE, October 1970, page 1445). It is known that lithium-niobate crystals (Li Nb O.sub.3) for an equal degree of light modulation need a substantially lower voltage of modulation. This advantage is however counterbalanced by a substantially larger change of static birefringence of Li Nb O.sub.3 with temperature than for KDP, this type of birefringence being effective also for dynamic modulation. There have been, accordingly, numerous efforts to reduce the disturbing effects of changes in temperature on Li Nb O.sub.3 -modulators (see F. S. Chen, Proc. IEEE, 1970, P. 1443). One way was to cut the modulator crystal into two components with a halfwaveplate in between or a turn by 90.degree. of the second component with respect to the first one (see F. S. Chen, Proc. IEEE, 1970, P. 1446). These methods are helpful only if the spatial and temporal temperature distribution is equal for both crystal components.
One object of the present invention therefore, is to provide an electrooptical distance measuring apparatus with a crystal modulator having improved compensation of changes of temperature. Another object is to provide such apparatus with a low voltage electrooptic modulator. Another object is to produce a distance measuring apparatus with a high accuracy of measurement.
The present invention is directed toward satisfying these objects with an apparatus comprising a modulating means with an electrooptic crystal and a quarterwave plate. These are arranged so, that the modulated light traverses at first the crystal and then successively the quarterwave plate, twice the measured distance, the quarterwave plate again in reverse direction and finally again the crystal. This modulator is mounted with a polarizing beam splitter in front of the electrooptic crystal. The modulated light, on its first passage of the beam splitter, being linearly polarized, after its first and second passage of the quarterwave plate and the crystal will be separable from its source and may be directed to a detecting means.
These objects and many other advantages of the present invention will be readily apparent to one skilled in the pertinent art from the following detailed description of the preferred embodiments thereof and the claims when considered in conjunction with the drawings.